1. Field of the Invention
The present invention relates to electronic circuitry for starting a sequence of electrical load elements.
2. Discussion of Background
Some electrical devices, such as motors and incandescent lamps, draw significantly higher levels of current during the first few seconds after starting than they do in steady-state operation. Switchgear and wiring for this equipment must be sized to accommodate the maximum current drawn, rather than the steady-state value. Breaking a combined load into several segments and bringing each segment on line at a slightly different time, so that their high-demand starting periods do not overlap, can reduce the peak current level dramatically. This method has particular advantages in computer-controlled systems, where even brief power transients can interrupt the system.
Many common electrical devices have radically different characteristics at the beginning of operation than they do in steady-state operation. In some cases, the difference causes a brief "starting surge" in which the current demand of the device rises quickly to a high value, then drops to a moderate value as the device reaches its normal, steady-state operating conditions. In some cases the peak value of the current may be over ten times the steady state value. The duration of the surge may range from several minutes down to a fraction of a second.
The causes of starting surge are varied, and may include changes in device resistance, inductance, or other, more complex, mechanisms. For example, because electrons are scattered by lattice vibrations, the electrical resistance of most metals increases with temperature. A tungsten light-bulb filament may have only one-tenth the resistance at room temperature that it has at white heat. When first turned on, therefore, the bulb draws a high level of current until the filament heats up. Although less dramatic, because working temperatures are usually lower, the same effect is present in most resistance heaters.
An A.C.-driven solenoid device, such as a solenoid valve or relay, consists of a coil wound on a magnetic armature that has one or more moving parts. When de-energized, the armature is held by springs in a mechanically and magnetically "open" position. Current flowing in the coil creates magnetic flux which overcomes the spring force and pulls the armature to a "closed" position, increasing the magnetic permeability of the circuit, and hence the coil inductance. Since current in an A.C. coil is chiefly inductance-limited, the current drawn is high while the armature is open but drops to a much lower value once it is closed.
In another example, a turning motor generates a voltage, or "back" electro-motive force (E.M.F.) which is proportional to motor speed and which normally opposes the applied voltage. The current drawn is proportional to the difference between the applied voltage and the back E.M.F. At low speeds, as on startup, there is little back E.M.F. and hence the current may be several times that drawn at high speeds.
In converting A.C. to D.C., a power supply usually charges a large electrolytic capacitor to a voltage well above the rated output; D.C. is then drawn from the capacitor and regulated downward to the desired voltage. During startup, a larger current is drawn to charge this capacitor from zero voltage up to its working level.
Starting surge is thus very common, and account must be taken of it in the design of any electrical or electronic system. Wiring and switchgear must be specified for the maximum value, and not merely the steady-state value, of the current which will flow through them, and batteries, power supplies and generators, for the maximum which they will be called upon to produce.
Failure to take starting surge into account can cause transient power disturbances or, in extreme cases, total system failure. Modern, computerized control systems are especially vulnerable because even split-second power interruptions can interfere with critical operations or cause data loss.
Starting surge can be mitigated by placing current-limiting devices, such as thermistors, in the current path. Long-life light bulbs, for example, may contain such devices. Current limiting, however, almost always degrades the performance of a device to which it is applied, since the limiter typically remains in the circuit after start-up and, thus, continues to consume power and reduce the available voltage during steady-state operation.
A reliable and widely-applicable method of reducing starting surge or its impact on electrical systems, without compromising the performance of the devices drawing the surge currents, would increase both the economy and the reliability of any system of which it was a part.
Starting surge can be reduced dramatically by switching some one or some few devices on at a time, rather than many together, and allowing enough time between switching operations for a surge associated with one device to die away before the next begins. For example, a group of four devices, each drawing ten amperes on startup but only two during normal operation, draws a peak current of forty amperes if all four devices are switched at once. If, however, the four devices are switched on separately, allowing one to approach steady state before the next is turned on, the peak current will only be sixteen amperes, a mere forty percent of the peak value of simultaneous start-up.
Use of sequential power-up to minimize surge is not new; it has long been used in high-power equipment, particularly in motor control centers. Classically, sequential power-up takes place under the control of thermal delay relays. More recently, the task has often been assigned to programmable controllers, or to arrays of integrated circuits such as the MC14541 and LS7210 programmable timers, which can provide individual delays from a few milliseconds to several hours.
All of the foregoing approaches have disadvantages. Thermal relays are cheap, but they include both heating elements, which can burn out, and mechanical contacts, which can wear or become pitted by repeated arcing. Programmable controllers, on the other hand, are reliable but expensive, and require time and usually special training to set up. Individual timer chips are low in cost, but to control each successive load segment requires a separate chip plus a number of outboard, passive components, and networks that typically require custom design and fabrication.